Japanese Patent No. 58-202316(A), issued on Nov. 25, 1983 in the name of Shiyouichi Honda discloses a double-overhead camshaft valve train mechanism using upper and lower tappets to operate each of the intake valves and the exhaust valves. The lower tappet abuts the stem tip of the associated valve and is of conventional inverted bucket design without an adjusting shim. The upper tappet is disposed with its bottom end abutting the lower tappet with its upper end in contact with the cam lobe of the camshaft. The top surface of the upper tappet is square with the tappet's longitudinal center axis, however, the bottom surface is inclined or at an angle to the top surface of the tappet. As a result, the reciprocating operational axes of both the upper and lower tappets are disposed at an angle to each other.
The lower tappet of the above mentioned valve train mechanism is supported for reciprocating movement by a tappet guide bored within the cylinder head of the engine. The upper tappet is similarly supported for reciprocating movement in a separate tappet carrier which is fastened to the top of the cylinder head and has a cylindrical structure with the center axes of both its internal bore (defining the tappet guide) and the external cylindrical hub disposed offset from each other. The carrier is formed with an ear which is slotted in a section of an arc concentric with the cylindrical hub. A set-screw passes through the slotted ear and is threaded in a portion of the cylinder head structure. Adjustment of the valve lash is realized by loosening the set-screw and rotating the tappet carrier so as to shift the axis of the upper tappet with respect to the axis of the lower tappet and, thereby, change the effective length of the upper tappet. Afterwards, the set-screw is re-tightened to maintain the tappet carrier in the adjusted position.
One problem with the mechanism shown in the above-mentioned Japanese patent is that the diameter of the upper tappet must be larger than geometrically required to avoid the top edges of the tappet being engaged by the flanks of the cam lobe of the camshaft. The large diameter of the upper tappet also contributes to increase its weight and forces an increase of the diameter of the lower tappet. Following the rule of thumb that in cylindrical elements such as tappets the weight increases as the cube of the diameter, my calculations (based on the patent drawings) show that the mechanism would be 4.13 times larger than a simple 34 mm tappet. This increase in dynamic mass and inertia would force the use of a longer and stiffer spring with still more mass and considerable more force.
Another problem with the Japanese mechanism described above is that the upper tappet in contact with the camshaft cannot rotate on its own axis and has no top shim wear pad to allow rotation with respect to the camshaft. It is well known that inverted bucket tappets, just as any other cam following devices, should rotate or have hardened top shims capable of rotation so as to minimize wear during its contact with the camshaft lobe. In addition, the non-rotational upper tappet, having to contend with the sideways camshaft lobe forces plus the sideways forces generated by its lower skewed edge being urged by the much-stronger spring, will tend to generate extra heat and wear at its transversal contact points with its guide structure.